The nervous system is an extremely complex structure containing a diversity of neurons and their synaptic interconnections. The normal development and function of the nervous system is general, and the visual system in particular, cannot be understood without knowing how information is transmitted across synapses. In almost all cases, this transfer of information is mediated by release of a chemical substance, called a neurotransmitter. Knowledge concerning the biochemical function and cellular mechanisms of neutrotransmitters will be critical to understanding how information is transferred through the neural circuitry of the visual system. However, progress in understanding the biochemistry of synaptic transmission in the visual system has lagged behind anatomical and physiological advances. This proposal represents another step in our long-range effort to understand the involvement of synaptic biochemistry in the transduction and processing of the visual stimulus. The specific emphasis for study is the cholinergic system, specifically the enzymes for synthesis (choline acetyltransferase) and degradation (acetylcholinesterase) of the transmitter, acetylcholine. The method is that of quantitative histochemistry. Using this method, histologically defined samples as small as 20 Mum in linear dimension are dissected from tissues and assayed for activities of transmitter enzymes. Careful mapping of sample locations facilitates correlation of data with anatomical and physiological results. Since most of the structures of the visual system are layered, they are particularly well suited for studies using the quantitative histochemical approach. Underlying mechanisms of diseases involving the visual system can be understood only upon a framework of information about how the nervous system normally functions. By studying the quantitative distribution of enzymes of acetylcholine metabolism in retina, superior colliculus and optic tectum of different specieis and the quantitative changes in the enzymes during development, a contribution should be made toward providing such a basis. The biochemical method to be used is the one that can best approach the precision of morphological and physiological investigations.